Cast iron may be defined as an allow of iron with sufficient carbon to form eutectic on solidification. This minimum amount is about two percent by weight. Other elements, particularly silicon, are invariably present in commercial cast irons. Under certain conditions of cooling rate and other element composition, part or all of the carbon dissolved in a molten alloy will precipitate during or after solidification as particles of free graphite. These particles can be observed on a polished surface of the iron using a magnification of about 50 times or more.
When no free graphite is precipitated on solidification the resulting cast iron is called "white" iron and is very hard, abrasion resistant, and brittle because all of the carbon is present as a very hard compound, iron carbide. If free graphite is present as tiny flakes the iron is called "gray" iron which is machinable and is widely used industrially. If the free graphite is present as tiny, nearly perfect spheres the iron is called "ductile" or "nodular" iron and it is machinable, of higher strength than gray iron, and is ductile or deformable without fracture.
A third type of iron containing free graphite is malleable iron, wherein the graphite is in finely dispersed compact masses called "temper carbon" which is formed only upon heat-treating a cast iron that has solidified as a white iron free of graphite.
Flakes of graphite interrupt the continuity of the metallic matrix, which is essentially that of a steel, and decrease its ductility to nearly zero and divide its tensile strength by a factor of about three. However the positive attributes of better machinability, better wear resistance, better heat resistance, and lower modulus of elasticity are thereby created.
If the free graphite is present not as flakes but as minute irregular masses, as in malleable iron, the machinability is excellent but tensile strength, ductility, and modulus of elasticity are increased to 70 to 80 percent of those of steel with the same matrix. If the free graphite is present as well-defined smooth spheroids, as in ductile or nodular iron, strength and particularlity ductility are increased still further.
There is an intermediate structure of free graphite called "vermicular" or "compacted", which is part way between flakes and smooth spheroids. The mechanical properties are midway between those of flake graphite iron and of ductile iron. These relationships are described, for example, in a report entitled "Relation Between Mechanical Properties and Graphite Structure in Cast Irons" by Ruff and Doshi in "Modern Casting", June, July and August, 1980.
The essential point is that the mechanical and physical properties of the various types of cast iron are strongly affected by the amount, shape and distribution of the free graphite present. For a particular service, a particular combination of properties is optimum, and the iron to reliably have these properties must be produced to consistently have the necessary causative microstructure.
Control of the shape and distribution of free graphite particles has been by changes in processing conditions, such as solidification rate, and in chemical composition, for example use of elements such as silicon and sulfur. Very small amounts of a wide range of other elements can have major effects on graphite morphology. For example, the presence of 0.02 percent of magnesium in the molten iron can cause the graphite to precipitate as spheres on solidification; but the presence of 0.002% of lead will prevent this effect. The mechanism by which these effects occur is not understood, but the technology is well enough known from experience for most practical purposes. Since theory is not clear nor agreed upon, in various circumstances it is not possible to predict an effect, or to clearly explain an observed effect. There is, therefore, a considerable element of empiricism in the important question of control of free graphite shape in the various cast irons.
Another factor in the structure and properties of the cast irons is the relative amounts of ferrite and pearlite which constitute the metallic matrix in which the graphite particles are embedded. Ferrite is iron which is free of carbon except for a very small dissolved amount, and so is soft, relatively low in strength, and very malleable or ductile. Pearlite is iron in which thin parallel plates of iron carbide are embedded, so that a grain of pearlite is three or four times as strong as ferrite, is harder, and considerably less malleable. Grains of pearlite and of ferrite are intermingled, the total being 100 percent of the metallic matrix, and the relative proportions strongly affect the hardness, strength, and ductility of the iron as a whole, independently of free graphite embedded.
Pearlite is formed by a solid state transformation during cooling of the iron through the temperature range of approximately 820 to 700 degrees C. The amount formed, from 0 to 100 percent of the matrix, is a function of cooling rate and iron composition, particularly silicon content, and also of the amount and distribution of free graphite present. Structures other than pearlite or ferrite can be formed such as martensite though these are not germane to the present development. The quantitative effects of cooling rate and composition, particularly major elements such as silicon and manganese, are well established. However, very small quantities of elements such as antimony can have strong effects on the amount of pearlite formed, and the quantitative effects of a wide range of trace elements are not known. In this respect the state of lack of knowledge is similar to that concerning graphite particle shape, although there is no necessary connection between the two phenomena.
Cast irons normally contain a long list of elements in trace amounts from 0.001 to 0.01 percent which originate in the ores from which the iron was originally smelted, and from remelted scrap which contains contaminants. The effects of these elements on the shape of graphite particles and on pearlite formation, to the extent where experience does not exist and theory cannot explain, is called the "heredity effect". It is a real condition which is present to a greater or lesser extent and which in some cases can have a large or even controlling effect on microstructure. The presence of these elements may explain why many cast irons will possess a graphite flake structure that is a mixture of types.
As noted above a wide range of combinations of mechanical and physical properties of the cast iron is possible by suitably adjusting the amount of free graphite in the iron, its particle shape and distribution, and the nature of the steel-like matrix in which it is embedded. For example, for making ingot molds into which molten steel is cast to solidify before being rolled or forged, it was found long ago that the best material is a flake gray cast iron with a large amount of graphite flakes (in the range of 3.6 to 4.5 percent total carbon content by weight, silicon in the range of about 1.0 to 2.5 percent, and manganese in the range of about 0.30 to 1.5 percent by weight) embedded in a matrix of mixed pearlite and ferrite, mostly ferrite. Experience has also shown that service life of molds is increased if the graphite flakes are more uniform in size and shape and are randomly distributed. It is believed that this is due to the lower modulus of elasticity thereby conferred. However, it is difficult and uncertain with the raw materials generally available to reliably and economically make a high carbon gray iron with graphite flakes in such favorable distribution.
Ingot molds have been made of ductile iron but have not been commercially successful since their increased cost is not accompanied by sufficiently increased service life. Malleable iron is not suitable for ingot molds since it cannot be made in the heavy section thicknesses necessary.
It has been discovered and published that if ingot molds are made of cast iron with vermicular or compacted graphite, their service life is much improved and they are now an article of commerce. There is no agreed explanation of the connection between microstructure and service life, but it is important that the graphite distribution be vermicular as defined by industry standards, with as little flake graphite or spheroidal graphite as possible. Such iron is peculiary difficult to consistently or reliably make, since the graphite structure is a midway point between flake and spheroidal achieved by addition of an optimum amount of special alloys including usually magnesium. The effect of other elements of composition and of "heredity" as noted above are particularly strong and therefore hard to predict and control.
Heretofore the raw materials from which commercial cast iron has been made have been remelted pig iron from blast furnaces, remelted iron and steel scrap, or their mixtures. Due to variations in composition of ores used and variations in blast furnaces operating practices, pig irons are known to vary considerably in the type and distribution of graphite flakes for the same composition and this follows through on remelting; and for obvious reasons there is considerable variation in remelted scrap because of adventitious other elements brought in. Such remelted irons, i.e. cast iron generally produced, are not easy to make with predictable structures and generally depend on known experiences with materials from known sources.
Pre-reduced iron is being produced in increasing quantities for making steel in arc furnaces, but very little has been available on the open market. It is characterized by being made from iron ore without reaching or even approaching the molten state. Limited experimental trials have been using pre-reduced iron to replace steel scrap in making cast iron, as reported for example in "Melting Prereduced Iron Ore Pellets in the Cupola" by Hofner et al, Transactions American Foundrymen's Society Vol. 76 (1968) page 53. However, to my knowledge there has been no commercial usage of it to make cast iron.
In order to determine suitability of pre-reduced iron pellets for making cast iron on a commercial scale, I initiated experiments in melting rate, losses, power consumption, and other production data and had test slabs cast to check on the mechanical properties of the cast iron made. On examining the microstructures of test bars, I discovered to my surprise that the graphite flakes were unusually straight, uniform, and free of mixed structure, and the pearlite content of the matrix was much lower than would be expected from the chemical corporation. Detailed comparison was then made of structure and mechanical properties of these samples and of samples taken from ingot molds of nearly identical composition made directly from blast furnace iron. This showed clearly the marked improvement accompanying the use of the pre-reduced iron as raw material. To my knowledge, this effect has not been previously known and I have found no reference to it in the literature. I believe it to be connected with the fact that the raw material has not been previously melted, but detailed explanation is not currently possible.
The advantages of the more uniform graphite distribution in the cast iron microstructure are at least two: One is that the more uniform geometry will give a lower modulus of elasticity to the iron especially at high carbon content, and this has been shown by comparative measurement. This property is of particular importance in products such as ingot molds and brake discs, in ingot molds probably increasing their service life by increasing their resistance to cracking.
A second advantage is that a base iron with such a propensity to cleanly defined graphite shapes will not only give a more reproducible gray iron structure, but will provide a more constant base for reproducible vermicular graphite structure upon suitable special alloy additions.
The increased proportion of ferrite to pearlite is mainly of economic importance in that it permits the use of decreased silicon content to arrive at a desired final ferrite content.
In summary, it is the purpose of my invention to improve the control which may be exercised over the microstructure of cast iron produced commercially in the foundry so that more uniform, clean, regular graphite particle shapes and distribution may be more reliably produced either in basic microstructures or in intentionally altered structures such as nodular cast iron or compacted graphite iron having a vermicular type microstructure. A more specific object is to improve the microstructure of ingot molds and other products made of cast iron.
This is accomplished by melting a charge composed largely of pre-reduced iron pellets to produce a cast iron containing approximately 3.6 to 4.5 percent total carbon content for heat resistant cast irons and 2.5 to 4.0% total carbon for other applications.